The present invention relates generally to the continuous casting of molten metal, such as molten steel, and more particularly to preventing undissolved alloying ingredients denser than the molten metal from entering the continuous casting mold.
In the continuous casting of molten steel, a stream of molten steel is poured from a ladle into an intermediate vessel known as a tundish having a bottom containing outlet openings through which molten steel flows into a continuous casting mold.
The tundish is composed of a metal shell having a bottom and an opening in the bottom. Refractory material lines the interior of the shell bottom to form a tundish interior bottom, and there is a first interface between the shell bottom and the refractory lining.
A vertically disposed nozzle element, separate and discrete from the shell and the lining, extends through the refractory lining and the opening in the shell bottom. The refractory material surrounds at least a major part of the nozzle element, and there is a second interface between the refractory material and the nozzle element.
The continuous casting mold is located below the nozzle element for receiving molten metal flowing downwardly through the nozzle element.
Free machining steels contain lead and/or bismuth to improve the machinability of the steel. Typical contents for each are about 0.04-0.40 wt. % bismuth and 0.05-0.50 wt. % lead.
Lead or bismuth may be added to the stream of molten steel entering the tundish. Lead and bismuth have a relatively low solubility in molten steel, compared to other alloying ingredients added to molten steel, and lead and bismuth are denser than molten steel. Because of these properties, substantial amounts of undissolved lead and bismuth tend to accumulate at the bottom of the tundish. For purposes of discussion, reference will hereafter be made to lead alone, but the problems and solutions applicable to lead described herein are also applicable to bismuth.
It has been determined that, one way or another, liquid lead finds its way to either or both of the first or second interfaces in the tundish, and from there the lead weeps or drips out through the bottom of the tundish, with much, if not most, of the liquid lead drippings entering the continuous casting mold, and that is undesirable because it can have an adverse effect on the quality of the cast steel product, providing undesirable lead globules in the cast steel. Lead weeping also results in decreased recovery of the lead added to the steel, as well as being a health hazard.
The metal tundish shell is normally provided with a plurality of bottom weep holes spaced from the bottom opening in the tundish shell through which the nozzle element extends. The purpose of the weep holes is to drain moisture which may accumulate at the bottom of the tundish shell. This moisture originates in the refractory lining for the tundish shell, and the moisture accumulates when a new refractory lining dries. However, with regard to those weep holes which overlie the casting mold, liquid lead which finds it way to the interface between the tundish shell bottom and the refractory lining adjacent the weep holes, can drain through these weep holes into the casting mold. The weep holes through which liquid lead can drip into the casting mold are those which are nearest to the tundish shell's bottom opening through which the nozzle element extends.
The second interface, i.e., the interface between the nozzle element and the adjacent refractory material, defines a downwardly extending seepage path along which liquid lead can seep toward the casting mold.
Located directly below the nozzle element and communicating therewith is a flow gate for controlling the flow of molten metal from the tundish through the nozzle element to the casting mold.
The aforementioned Jackson et al. patent application, of which this application is a continuation-in-part, was directed to the problem of preventing undissolved lead which accumulated on the tundish interior bottom from being carried out through the outlet openings or nozzle elements in the tundish.